Structure Evolution and Hydrogenation Performance of IrFe Bimetallic Nanomaterials

Article abstract of DOI:10.1021/acs.langmuir.5b04566

By a reverse microemulsion method, a series of IrFe bimetallic nanomaterials of variable morphologies and compositions is synthesized and characterized by ⁵⁷Fe M?ssbauer spectroscopy, XRD, XPS, and TEM. The structure evolution, such as IrFe alloy nanoparticles to Ir nanoparticles on Fe₂O₃ flakes, can be simply tuned by changing the molar ratio of Ir to Fe precursors. In terms of Fe, the relative content of IrFe alloy decreased with the increase of Fe species doped, while that of Fe₂O₃ flakes increased until reached 100%. The as-prepared IrFe bimetallic nanomaterials were served as catalysts for the selective hydrogenation of 3-nitrostyrene to 3-aminostyrene, and it is found that the catalytic performance was related to the morphology and composition of these nanomaterials. Ir₁Fe₄ was subsequently identified to be a highly active and exceedingly selective catalyst with good stability and recyclability for the hydrogenation of 3-nitrostyrene, underscoring a remarkable "synergistic effect" of the two metals appearing as the form of Ir nanoparticles loaded on Fe₂O₃ flakes. For Ir nanoparticles, they act as an active species for the hydrogenation; for Fe₂O₃ flakes, they favor the preferential adsorption of nitro groups, which account for the better chemoselectivity to objective product.

Full text of DOI:10.1021/acs.langmuir.5b04566

Article
pubs.acs.org/Langmuir
Structure Evolution and Hydrogenation Performance of IrFe
Bimetallic Nanomaterials
Ting Lu, Jian Lin, Xin Liu, Xiaodong Wang,* and Tao Zhang
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian
116023, P.R. China
ABSTRACT: By a reverse microemulsion method, a series of IrFe
bimetallic nanomaterials of variable morphologies and composi-
tions is synthesized and characterized by 57Fe Mossbauer
̈
spectroscopy, XRD, XPS, and TEM. The structure evolution,
such as IrFe alloy nanoparticles to Ir nanoparticles on Fe2O3 flakes,
can be simply tuned by changing the molar ratio of Ir to Fe
precursors. In terms of Fe, the relative content of IrFe alloy
decreased with the increase of Fe species doped, while that of
Fe2O3 flakes increased until reached 100%. The as-prepared IrFe
bimetallic nanomaterials were served as catalysts for the selective
hydrogenation of 3-nitrostyrene to 3-aminostyrene, and it is found
that the catalytic performance was related to the morphology and
composition of these nanomaterials. Ir1Fe4 was subsequently
identified to be a highly active and exceedingly selective catalyst
with good stability and recyclability for the hydrogenation of 3-nitrostyrene, underscoring a remarkable “synergistic effect” of the
two metals appearing as the form of Ir nanoparticles loaded on Fe2O3 flakes. For Ir nanoparticles, they act as an active species for
the hydrogenation; for Fe2O3 flakes, they favor the preferential adsorption of nitro groups, which account for the better
chemoselectivity to objective product.
ticles.12,17,18 In our previous work, we have successfully
synthesized Ir nanowires and studied their excellent catalytic
INTRODUCTION
■
Inorganic nanostructures, which have at least one dimension
performance on the selective hydrogenation of o-chloroni-
between 1 and 100 nm by definition, have attracted immense
trobenzene.19 Functionalized anilines are important intermedi-
interest in their syntheses, owing to their fascinating properties
in both fundamental research and practical application.1−3
ates for the manufacture of many agrochemicals, pharmaceut-
icals, dyes and pigments.20−23 Although the hydrogenation of
Nanoparticles in their nanoscale dimensions exhibit unique
simple aromatic nitroarenes to corresponding aniline deriva-
electronic, spectroscopic and chemical properties due to their
small size and high surface-to-volume ratio.4,5 The chemical and
tives poses few problems and is carried out catalytically on very
large scales, it is still difficult to catalytically reduce the nitro
physical properties so much as their functionality of the
nanomaterials are strongly dependent on the size, shape, and
group in a selective manner or needs harsh reaction conditions
architecture of the nanostructures.6,7 Moreover, compared with
when the reactant contains other reducible functional groups
(e.g., CC, CC, CO, CN, etc.). The good chemo-
single metal, bimetallic nanomaterials, which can be a random
selective hydrogenation of Ir nanowires on o-chloronitroben-
alloy, a core−shell structure, or just mixed monometallic
zene has inspired us to explore Ir nanowires in the
nanoparticles, usually exhibit performance superior to their
hydrogenation of nitroarenes with other reducible functional
monometallic counterparts, which therefore have attracted a
groups. However, it was found that both −NO2 and −CC
great deal of attention from both academic and industrial
fields.8−10
groups were reduced in the hydrogenation of 3-nitrostyrene
over Ir nanowires, suggesting that Ir nanowires exhibit no
chemoselectivity to 3-aminostyrene. However, as mentioned
above, Ir-based catalysts were usually used in many catalysis
reactions, consequently showing high expectations to utilize the
Recently, nanoscale metal particles have been widely used as
effective catalysts because of their high density of active
sites.11,12 Among different noble metals, iridium nanoparticles
were found outstanding candidates owing to their high catalytic
Ir nanowires in the selective hydrogenation of nitroarenes.
activity, stability and selectivity in many catalysis reactions like
alkene hydrogenation,13 CO oxidation,14 oxygen reduction
Thus, how to realize the chemoselective hydrogenation of the
reaction,15 and for direct methanol fuel cell reaction.16
nitro group to the desired 3-aminostyrene with the preservation
However, compared with other noble metals, the synthesis of
size and shape controlled Ir nanostructures remains a challenge;
there were only a few reports on the shape control of Ir
nanomaterials, and most of them limited to nanopar-
Received: December 14, 2015
Revised: March 2, 2016
© XXXX American Chemical Society
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Accordingly, 57Fe Mossbauer spectral parameters such as the isomer
̈
of CC double bond, especially under mild reaction
conditions is now the problem we faced.
shift (IS), the electric quadrupole splitting (QS), the full line width at
half-maximum (fwhm), and the relative spectral area (A) of different
components on the absorption patterns were calculated. The IS values
were quoted relative to α-Fe at room temperature.
To solve this problem, many strategies are commonly
desirable, based on the catalyst itself or introducing another
component. It has been demonstrated that the selectivity of
hydrogenation by monometallic nanoparticles can be improved
by doping a selected metal.24,25 Among these methods,
bimetallic nanoparticles, which are widely applicable to
heterogeneous transformations, frequently exhibit catalytic
performance superior to their monometallic counterparts, a
phenomenon referred as “synergistic effect”.9,26 For example,
Pd−Ag bimetallic systems are widely used as selective
hydrogenation catalysts,27 Fe-doping Rh/Fe nanoparticle
catalyst shows dramatically enhanced catalytic performance
for nitroarene hydrogenation,28 and Ir−Fe system is more
amenable than either Pt−Fe or Pd−Fe as redox sensors for use
in hydrothermal experiments.29 Thus, it is expected that the
introduction of another metal into Ir precursor can improve the
performance in selective hydrogenation reaction.
In this Article, we have facilely synthesized a series of IrFe
bimetallic nanomaterials by reverse microemulsion technique
and studied the effects of Fe doping on the structure evolution
and catalytic performance of Ir precursor. According to the
chemical valence of Fe and the morphology of the materials,
the bimetallic nanostructures can be classified as single IrFe
alloy nanoparticles, IrFe alloy nanoparticles coexisting with
Fe2O3 flakes, or Ir nanoparticles loaded on Fe2O3 flakes. These
bimetallic nanomaterials are explored as highly active and
selective catalysts in the hydrogenation of 3-nitrostyrene under
mild conditions.
Catalytic Testing. The hydrogenation of 3-nitrostyrene was
conducted in an autoclave equipped with a pressure control system.
In each reaction, a mixture of IrFe catalyst (10 mg), inert SiO2 (90 mg,
which was used just to eliminate the effect of reactant diffusion due to
the small amount of catalyst), reactant 3-nitrostyrene (0.5 mmol),
internal standard o-xylene (0.25 mmol) and solvent toluene, in a total
volume of 5 mL was placed into the autoclave. When the autoclave was
sealed, air was purged by flushing with high pressure hydrogen more
than 10 times. Then, the initial H2 pressure was adjusted to 3 bar and
the autoclave was equilibrated in a thermostatic bath at 40 °C. A
magnetic stirring of 1200 r/min which is large enough to eliminate the
external diffusion effect was carried out to initiate the reaction when
the temperature of the thermocouple inserted was up to 40 °C. The
H2 pressure decreased gradually during the reaction as a result of
hydrogenation. The reaction time was determined after many times
experiments to the maximum of the conversion. After reaction, the
product was analyzed by GC.
RESULTS AND DISCUSSION
■
Nanomaterials Characterization. 57Fe Mossbauer spec-
̈
troscopy was employed to characterize the chemical state of
iron in IrFe bimetallic nanomaterials. As seen from Figure 1
EXPERIMENTAL SECTION
■
Catalyst Preparation. Microemulsion was first fabricated by
CTAB/water/n-heptane/n-hexanol with CTAB (cetyltrimethylammo-
nium bromide, purchased from SIGMA-Aldrich Co. with purity
≥99%) weight of 2.0 g (0.17 mol/L), n-heptane volume of 20 mL, and
n-hexanol volume of 12 mL, which was equilibrated at 60 °C. Then,
aqueous solutions of H2IrCl6 (0.5 mol/L) and FeCl3 (0.5 mol/L) at
various desired molar ratios and total 1 mL volume were respectively
added with continual stirring. The solution turned into dark brown
color, which was still heated at 60 °C and stirred at least for 2 h. After
that, 1.5 mL of freshly prepared 2 mol/L NaBH4 aqueous solution was
added dropwise under vigorous stirring. The color of the solution
quickly turned from dark brown to black. Finally, the resultant mixture
was incubated for 2 h with continuous stirring before being repeatedly
washed with ethanol and water at least three times, respectively. The
crude product was then retrieved by centrifugation and dried in a
vacuum at 80 °C for 8 h. The used Fe2O3 and Ir nanoparticles in Table
2 were purchased from Aladdin Chemistry Co. Ltd. with 99.5% purity
and prepared according to the published paper,19 respectively.
Catalyst Characterization. X-ray diffraction (XRD) patterns were
collected on a PANalytical X’pert diffractometer equipped with Cu Kα
radiation (λ = 1.54184 Å), operated at 40 kV and 40 mA. The
morphologies of the samples were observed with transmission electron
microscopy (TEM) using a JEM-2100F (JEOL) electronic microscope
with an accelerating voltage of 200 kV. Before TEM experiments, the
samples were ultrasonically dispersed in ethanol and then a drop of the
solution was put onto a copper grid coated with a thin holey carbon
film. The binding energies of surface species were determined by X-ray
photoelectron spectra (XPS) on an ESCALAB250 X-ray photo-
electron spectrometer with contaminated C as internal standard (C 1s
= 284.5 eV) and special agglutinant containing the elements C, O and
Figure 1. Room temperature 57Fe Mossbauer spectra for IrFe
nanomaterials at various Ir/Fe molar ratios: (a) 4/1, (b) 2/1, (c) 1/
1, (d) 1/2, (e) 1/4, (f) spent 1/4, and (g) 0/1.
̈
and Table 1, when Ir to Fe molar ratio was 4/1, a singlet peak
was observed with IS value of 0.05, which is very close to that of
Fe0. Combined with the XPS binding energy of Ir 4f7/2 (60.7
eV, Figure 2b derived from Figure 2a), which corresponds to
Ir0, it is indicated that Fe species was present as IrFe alloy
without any protection (following XRD result further
confirmed this view). It should be mentioned that although
the XPS peak of Fe 2p3/2 at 711 eV (Figure 2c) corresponding
to Fe3+ was examined for Ir/Fe = 4/1 sample, it was very weak
and almost could be ignored. It may be attributed to the
Si, was used to fix the sample and sampler. 57Fe Mossbauer spectra
̈
were recorded using a Topologic 500A spectrometer and a
proportional counter at room temperature. 57Co(Rh) moving in a
constant acceleration mode was used as radioactive source.
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Table 1. Room Temperature 57Fe Mossbauer Parameters of IrFe Nanomaterials at Various Ir/Fe Molar Ratios
̈
a
b
c
entry
Ir/Fe (mol/mol)
subspectra
oxidation state of iron
IS (mm/s)
QS (mm/s)
spectral area (%)
fwhm (mm/s)
1
2
4/1
2/1
singlet
Fe0
0.05
0.05
0
100
69.3
30.7
28.7
71.3
6.9
0.79
0.69
0.48
0.65
0.55
0.31
0.54
0.53
0.58
0.42
singlet
Fe0
0
doublet
singlet
Fe3+
Fe0
Fe3+
Fe0
0.28
0.97
0
3
4
5
1/1
1/2
−0.01
0.25
doublet
singlet
0.93
0
−0.04
0.23
doublet
doublet
doublet
doublet
Fe3+
Fe3+
Fe3+
0.89
0.79
0.77
0.71
93.1
100
100
100
1/4
0.24
d
6
1/4 (spent)
0/1
0.27
7
Fe3+
0.24
a
b
c
d
δ, isomer shift, related to α-Fe. QS, electric quadrupole splitting. Uncertainty is 5% of reported value. Entry 6 lists the room temperature 57Fe
Mossbauer parameters of spent Ir Fe nanomaterial after five cycles of 3-nitrostyrene hydrogenation.
̈
1
4
Figure 2. XPS spectra for IrFe nanomaterials at Ir/Fe molar ratios of 4/1 and 1/4 before and after five cycles hydrogenation: (a) surveys; (b) Ir
windows; (c) Fe windows.
oxidation of little Fe exposed on the sample surface which was
not alloy with Ir. Because the amount of this oxidized Fe3+ was
very few, it is less than the error range and cannot be examined
at Ir/Fe = 1/4 and Fe species existed as a form of Fe3+ (Figure
2c). Combined with Mossbauer measurement results, it can be
̈
concluded that, with the doping of Fe species into Ir precursor,
not only the chemical states of Ir and Fe species but also the
relative proportions of different Fe species are modulated.
Specifically, with the increase of Fe content in IrFe bimetallic
nanomaterials, the Ir and Fe species changed from 100% IrFe
alloy to the coexistence of IrFe alloy and Fe2O3, and finally to
single metal Ir coexisting with Fe2O3. Detailed morphologies
are characterized in the following text.
by Mossbauer spectroscopy preferring to the bulk property.
̈
When Ir/Fe = 2/1, besides the IrFe alloy singlet peak, a new
doublet subspectrum corresponding to Fe3+ (Fe2O3) appeared,
and the relative spectral area was 30.7%. Further increasing Fe
content from Ir/Fe = 1/1 to 1/2, the relative content of IrFe
alloy decreased from 28.7% to 6.9%, while that of Fe3+
increased from 71.3% to 93.1%. Particularly, when Ir/Fe
reached 1/4, the state of Fe species in IrFe bimetallic
nanomaterials was approximate to that of sole Fe3+ sample
(Ir/Fe = 0/1), suggesting that IrFe alloy absolutely disappeared
and Fe species existed as 100% Fe3+. Meanwhile, for Ir species,
XPS results (Figure 2b) revealed it still existed as a form of Ir0
Figure 3 shows the XRD patterns of IrFe bimetallic
nanomaterias at various Ir/Fe molar ratios. In the case of the
sole Fe sample (Figure 3a(8)), the major diffraction peaks with
2θ values of 35.6° and 62.8° can be indexed to the (311) plane
and (440) plane of maghemite Fe2O3 (PDF #00-024-0081).
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Figure 3. Whole (a) and local magnified (b) XRD patterns for IrFe nanomaterials at various Ir/Fe molar ratios: (1)1/0, (2) 4/1, (3) 2/1, (4) 1/1,
(5) 1/2, (6) 1/4, (7) spent 1/4, and (8) 0/1.
Figure 4. TEM and HRTEM images for IrFe nanomaterials at various Ir/Fe molar ratios: (a) 1/0, (b) 4/1, (c) 2/1, (d) 1/2, (e) 1/4, and (f) 0/1.
For all the IrFe bimetallic nanomaterials with different Ir-to-Fe
ratios, two main diffraction peaks were still related to sole Ir
sample (Figure 3a(1), PDF #00-006-0598). Moreover, a
diffraction peak at about 22.2° assigned to the amorphous
SiO2 was present for all the samples with different intensities,
which was induced by the sample platform containing SiO2.30 It
is also found that with the increase of Fe content, the noise of
baselines increased, which probably induced by the increase of
amorphous part linked to Fe2O3. Notably, as the content of Fe
increased from Ir/Fe = 4/1 to Ir/Fe = 1/2, the peaks for
Ir(111) (original 41.1°, Figure 3b(1)) shifted to 41.3°, 41.5°,
41.7°, and 41.9°, respectively (Figure 3b(2)−(5)). This shift in
2θ values could be attributed to the formation of IrFe alloy,
Interestingly, when Ir/Fe = 1/4 (Figure 3a(6)), no obvious
peak shift can be observed, indicating that IrFe alloy was nearly
no longer formed. Instead, two small Fe2O3 peaks (311) and
(400) besides the characteristic Ir(111) were detected, in
accordance with the Mossbauer result that only Fe3+ without
̈
IrFe alloy existed at Ir/Fe = 1/4.
TEM characterization was performed and representative
images are shown in Figure 4. For pure Ir, one-dimensional Ir
nanowires with 2 nm diameter were obtained (Figure 4a).
When doping a little amount of Fe species into Ir precursor,
nanoparticles but not nanowires were observed at Ir/Fe = 4/1
(Figure 4b), which possessed 5 nm size. The formation of
nanoparticles may be attributed to the specific interactions
among IrFe nanocrystals and surfactant molecules arising from
which is in good agreement with the Mossbauer results.
̈
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Figure 5. STEM and EDS mapping images and bulk elemental analysis for IrFe nanomaterials at Ir/Fe molar ratios of 4/1 (a) and 1/4 (b,c).
Fe species doping, which restricts the growth of surfactant-
encapsulated primary nanoparticles into nanowires. Combined
distribution dots. It can be attributed to that at Ir/Fe = 1/4,
Fe species formed continuous Fe2O3 flakes while Ir species
presented as 2 nm nanoparticles distributed on Fe2O3 flakes
randomly. Moreover, the selected-area bulk elemental analysis
(Figure 5c) showed no other peaks expect for C, O, Ir, Fe and
Cu (derived from copper grid) can be detected, indicating the
disposal of reduced agent and surfactant. And the bulk
composition of Ir/Fe was close to 1/3.2, which deviated only
a little from the designed composition of Ir/Fe = 1/4. When
the doping amount of Fe species reached the ultimate value,
that is, Ir/Fe = 0/1, thin flakes with obvious crinkles were
obtained by TEM observation (Figure 4f). That is to say, for
pure Fe species, it existed as thin Fe2O3 flakes totally by our
reverse microemulsion method.
with the Mossbauer and XRD results (Figures 1a and 3a(2)),
̈
these nanoparticles were IrFe alloy nanoparticles. HRTEM
image in the inset of Figure 4b particularly verified the
formation of IrFe alloy that the observed plane spacing of 2.11
Å deviated from the major component Ir of the most obvious
crystal facets (111) (2.20 Å). Element Ir and Fe EDS mappings
for Ir/Fe = 4/1 (Figure 5a) also showed the uniformity and
synchronicity of Ir and Fe distribution. Further increasing the
doping amount of Fe species to Ir/Fe = 2/1 (Figure 4c),
besides nanoparticles, some thin flakes were dimly visible. With
the increase of Ir/Fe = 1/2, the flakes looked more clearly on
which still loaded some nanoparticles (Figure 4d). Based on the
Mossbauer measurement results, molar Fe species distributed
According to the above results, it could be concluded that
doping of Fe species into Ir precursor can change and modulate
the existing form, morphology and distribution of both Ir and
Fe species. From the viewpoint of H2IrCl6 microemulsion, the
addition of Fe3+ not only increases the ionic strength of the
solution, but also compresses the double layer around the polar
headgroups and screens the electrostatic repulsion between the
charged headgroups of CTAB. Thus, the area per headgroup of
the surfactant molecule is decreased, resulting in the increase of
the critical packing parameter p31−33 which is beneficial to the
formation of larger assemblies such as reverse micelles (p is
defined as v/a0lc, where v is the hydrophobic portion of the
surfactant, a0 is the equilibrium area per molecule at the
aggregate surface, and lc is the length of the hydrophobic group;
0 ≤ p ≤ 1/3 for spherical micelles, 1/3 ≤ p ≤ 1/2 for
cylindrical micelles, 1/2≤ p ≤ 1 for bilayer structures, and p > 1
for inverted micelles). Moreover, the addition of Fe3+ would
influence the specific interactions among IrFe nanocrystals,
surfactant molecules and excess NaBH4, preventing the
̈
as 69.3% Fe0 and 30.7% Fe3+ at Ir/Fe = 2/1, and 6.9% Fe0 and
93.1% Fe3+ at Ir/Fe = 1/2. Thus, it can be concluded that the
nanoparticles corresponded to 5 nm IrFe alloy nanoparticles
and the flakes assigned to thin Fe2O3 flakes at Ir/Fe = 2/1 and
1/2. Further increasing Ir/Fe = 1/4, the flakes still can be
observed clearly, on which 2 nm smaller nanoparticles other
than 5 nm IrFe alloy nanoparticles were loaded (Figure 4e).
According to Mossbauer and XPS results, the Fe species was
̈
totally Fe2O3 and the Ir species was single metal Ir at Ir/Fe = 1/
4, thus it can be deduced that Fe species presented as Fe2O3
flakes and Ir species existed as nanoparticles with size of 2 nm
supported on Fe2O3 flakes. Meanwhile, the statistical plane
spacing of 2.22 Å (inset in Figure 4e) just corresponded to
Ir(111). Compared with EDS mapping of element Ir (Figure
5b), that of element Fe looked uniform without any obvious
gap in the regions marked by yellow and blue circles where
there were nearly no Ir nanoparticles observed according to
STEM image; while for element Ir, there were a few
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a
Table 2. Hydrogenation of 3-Nitrostyrene over IrFe Nanomaterials at Various Ir/Fe Molar Ratios
f
entry
Ir/Fe (mol/mol)
morphology
reaction time (min)
conversion (%)
selectivity to 2a (%)
TON
b
1
2
3
4
5
6
7
8
9
1/0
4/1
2/1
1/1
1/2
1/4
0/1
0/1
1/4
Ir NWs
15
24
98
95
91
83
86
93
0
2
3
9.4
9.8
c
IrFe NPs
IrFe NPs on Fe2O3 flakes
IrFe NPs on Fe2O3 flakes
IrFe NPs on Fe2O3 flakes
Ir NPs on Fe2O3 flakes
Fe2O3 flakes
19
71
90
96
92
0
10.0
10.3
13.1
19.3
0
135
195
105
420
240
12
d
Fe2O3 NPs
0
0
0
e
Ir NPs + Fe2O3 flakes
99
0
25.4
a
Reaction conditions: T = 40 °C, p = 3 bar; 10 mg of catalyst + 90 mg of SiO2, 0.5 mmol 3-nitrostyrene, 5 mL of toluene solvent, o-xylene as internal
b
c
d
standard. NWs: nanowires. NPs: nanoparticles. Fe2O3 nanoparticles were purchased from Aladdin Chemistry Co. Ltd. with 99.5% purity and 10
nm size. Ir nanoparticles were prepared according to the published paper19 with 2 nm size. TON (turnover number), specific activity per Ir atom
e
f
for 3-nitrostyrene hydrogenation at the maximum conversion.
aggregation of surfactant-encapsulated primary nanoparticles
into nanowires. On the other hand, as an important synthesis
parameter, the Ir/Fe/NaBH4 ratio, plays a key role in regulating
the composition and morphology of the final IrFe bimetallic
products. As a versatile reducing agent, NaBH4 is frequently
used in a wide range of reduction process.34 In our synthesized
process, fixed and excess NaBH4 with 6 times molar ratios of
NaBH4 to H2IrCl6 and FeCl3 was used to ensure the complete
reduction of Ir and Fe species. With the variation of H2IrCl6 to
FeCl3 molar ratios, the morphology of IrFe nanomaterials
changed from entire 5 nm IrFe alloy nanoparticles to the
coexistence of 5 nm IrFe alloy nanoparticles and Fe2O3 flakes,
and finally to 2 nm Ir nanoparticles loaded on Fe2O3 flakes.
Based on Vegard’s law:35
observed with further increasing amount of Fe species (entries
3−6), that is, a good chemoselectivity toward the reduction of
nitro group, and afforded the desired 3-aminostyrene product
without the reduction of CC to 3-ethylaniline. The best
result was obtained with Ir1Fe4 (entry 6), over which both
conversion and selectivity achieved more than 90%. This
performance was comparable to those over Pt- and Fe2O3-
based catalysts.22,36 Moreover, the whole amount of Ir
component was used to calculate TON only for reference. As
shown, the TON values increased with the increasing content
of Fe in IrFe bimetallic nanomaterials, and the maximum value
of TON was observed over the mixture of Ir nanoparticles and
Fe2O3 flakes.
The kinetic studies were carried out to follow the variations
of conversion, yield and pressure versus time. Here, the yield
was defined as reactant conversion × product selectivity to 3-
aminostyrene listed in Table 2. As shown in Figure 6, the
pressure decreased gradually with the reaction time, and the
conversion and yield first increased quickly then became gently
and finally decreased after 105 min reaction over Ir1Fe4.
Control test over either Fe2O3 flakes or Fe2O3 nanoparticles
(entries 7 and 8 in Table 2) under identical reaction conditions
d0Ir Fe = xd0Ir + (1 − x)d0Fe
x
1−x
We made a calculation of the qualitative phase composition of
IrFe alloy in different Ir/Fe ratios using the peak with the
highest intensity in XRD (Figure 3b), where d0 are the lattice
constants for IrFe alloy, Ir, and Fe, which were calculated
through MDIJade5.0 software, and x is the molar fraction of Ir
in IrFe alloy. The alloys were Ir94Fe6 for Ir/Fe = 4/1 and 2/1,
Ir92Fe8 for Ir/Fe = 1/1, and Ir90Fe10 for Ir/Fe = 1/2. Clearly,
the phase composition of IrFe alloy contained at least 90% Ir
and the molar fraction of Fe less than 10%. That is to say, when
the relative amount of Fe species was small, a limited content of
Fe species enter the crystal lattice of Ir after the addition of
NaBH4 and satisfied the requirement of crystal lattice matching
to form IrFe alloy. When the relative content of Fe species was
large, the excessive Fe species no longer satisfied the crystal
lattice matching of IrFe alloy formation, thus the phase
separation of Ir and Fe species occurred, resulting in the
formation of Ir nanoparticles and Fe2O3 flakes, respectively.
Catalytic Activity. The catalytic performance of IrFe
nanomaterials was evaluated in the selective hydrogenation of
3-nitrostyrene under mild reaction conditions (3 bar, 40 °C), as
shown in Table 2. As the benchmark, pure Ir nanowires showed
good activity but very low selectivity to the desired product
2a36 (entry 1); nearly all the −NO2 and −CC were reduced.
With the addition of Fe, the catalytic performance of Ir4Fe1 was
found to be same as that of pure Ir nanowires (entry 2).
Interestingly, a considerable improvement in the selectivity was
Figure 6. Kinetic curves for 3-nitrostyrene hydrogenation with Ir1Fe4
under 40 °C and 3 bar.
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did not show any activity at all, highlighting the critical role of
Ir component.
The above-mentioned results demonstrated that on the basis
of keeping high conversion, the introduction of Fe species into
Ir precursor dramatically changes the chemoselectivity to the
objective product 3-aminostyrene. Combined the catalytic
activity with Mossbauer characterization (entry 1 in Table 1),
̈
it can be found that without Fe2O3 flakes formation, the
selectivity to objective product 2a was very low (entry 2 in
Table 2), indicating that the IrFe alloy nanoparticles hardly
make a contribution to the selectivity. With further increase of
Fe content, the relative content of IrFe alloy decreased and that
of Fe2O3 flakes increased; correspondingly, the selectivity to
objective product 2a increased. Clearly, the formation of Fe2O3
flakes makes a positive contribution to the selectivity. However,
this result did not mean that the contribution of Ir can be
ignored; instead, it plays an important role in the reaction.
Taking Ir1Fe4 for an example, the participation of Fe species
dramatically improved the selectivity of Ir nanowires from 2%
to 92%. It should be emphasized that the Ir nanoparticles and
Fe2O3 flakes did not separate from each other, but “synergistic
effect” existed between them. Physically mixing Ir nanoparticles
(2 nm, synthesized according to the previous paper19) with
Fe2O3 flakes with a molar ratio of 1/2 to simulate Ir1Fe4
composite in the hydrogenation reaction, it is found that,
similar to pure Ir nanowires or Ir nanoparticles, this mixture did
not possess the chemoselectivity toward the reduction of nitro
group and did not afforded the desired 3-aminostyrene product
at all (entry 9 in Table 2). The absolutely different
chemoselectivity between Ir1Fe4 and mixture of Ir nanoparticles
and Fe2O3 flakes proved that the Ir nanoparticles and Fe2O3
flakes are not absolutely dissociative but there is indeed some
synergistic effect affecting the nature of chemoselectivity. It is
also found from the XPS result (Figure 2b) that compared with
Ir4Fe1 (60.7 eV) no significant binding energy shift of Ir 4f7/2
can be observed in Ir1Fe4 (60.8 eV), indicating that the
interaction between Ir and Fe species in Ir1Fe4 was comparable
to that in Ir4Fe1 alloy.37 In other words, similar to the strong
interaction of Ir and Fe species in Ir4Fe1 alloy, there was also a
strong interaction between the components in Ir1Fe4. It is
proposed that the chemoselective hydrogenation of nitro
groups arises from the preferential and strong adsorption of
nitro groups on the specific sites of Fe2O3 flakes,38,39 and the Ir
nanoparticles played a major role in activating the hydrogen.23
The N2 adsorption/desorption results (Figure 7) just proved
the above viewpoint that the BET surface area of Ir1Fe4 (Ir
Figure 7. N2 adsorption/desorption isotherms of Ir4Fe1 and Ir1Fe4
nanomaterials.
Figure 8. Cycle performance for Ir1Fe4 nanomaterial.
spectroscopy, XPS, and XRD, as shown in Figures 1−3. It can
be seen from Figure 1(f) that although the quality of
Mossbauer spectrum looked poorer of spent Ir Fe , there was
̈
1
4
no obvious change on the Mossbauer parameters for the spent
̈
Ir1Fe4 compared with those for the fresh one. From XPS
spectra (Figure 2), the characteristic peak positions both for Ir
and Fe windows hardly shifted before and after the reaction.
For XRD pattern (Figure 3a(7)), there was also no obvious
shift on amorphous SiO2 and Ir peak positions, only the relative
intensities of them changed which was induced by the addition
of SiO2 in the reaction. Combined with the good cycle
performance, the above results confirmed the good stability of
this nanomaterial.
nanoparticles @ Fe2O3 flakes, 82.6
0.4 m2/g) was much
larger than that of Ir4Fe1 (IrFe alloy nanoparticles, 8.1 0.04
m2/g), which is consistent with their catalytic performance in
traditional understand. Meanwhile, it indicated that the
formation of Fe2O3 flakes was beneficial to the increase of
surface area compared with that of IrFe alloy nanoparticles.
The stability of IrFe bimetallic nanomaterials as a catalyst was
also explored. Taking the best Ir1Fe4 for an example, the IrFe
nanomaterial could be easily recycled by centrifugation, without
visible decay in activity and selectivity for at least five cycles
(Figure 8). After the fifth cycle, ICP-MS result showed that
there were no Ir and Fe species in the filtrate that can be
observed to the detectable limits, indicating that leaching does
not occur and the reaction is catalyzed exclusively by the
heterogeneous Ir1Fe4 composite, i.e. Ir nanoparticles loaded on
Fe2O3 flakes. Subsequently, the spent Ir1Fe4 nanomaterial after
CONCLUSION
■
In summary, a series of IrFe bimetallic nanomaterials of variable
morphologies and compositions is synthesized through doping
Fe into Ir precursor by a reverse microemulsion method. The
structure evolution of IrFe from entire 5 nm IrFe alloy
nanoparticles to the coexistence of 5 nm IrFe alloy nano-
particles and Fe2O3 flakes, and finally to 2 nm Ir nanoparticles
loaded on Fe2O3 flakes can be controlled by tuning the relative
ratio of Ir-to-Fe precursors. The above IrFe nanomaterials with
three kinds of microstructures are applied as catalysts in the
five cycles hydrogenation was characterized by 57Fe Mossbauer
̈
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DOI: 10.1021/acs.langmuir.5b04566
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Article
(10) Cai, S. F.; Duan, H. H.; Rong, H. P.; Wang, D. S.; Li, L. S.; He,
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Nanoparticles in the Hydrogenation of Nitroarenes. ACS Catal. 2013,
3, 608−612.
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reaction conditions to evaluate their hydrogenation perform-
ance. It is found that the catalytic activity follows the sequence
of Ir nanoparticles loaded on Fe2O3 flakes > IrFe alloy
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: xdwang@dicp.ac.cn. Tel: +86-411-84379680. Fax:
+86-411-84685940.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21003124, 21076211, 21203181, and
11205160).
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